Hi guys!
I have a problem trying to adjust the LM317 voltage with a digital pot (MCP41050, 50K). I have placed the pot in parallel with a 3,3K(R2) resistor and this makes me adjust the resistance from 3095ohm to about 270ohm+- (measured). The R1 resistor is 220 ohm. After what i have calculated, this will give me a range from bout 20V to 2.80V.

The problem is that im just able to adjust it from 2.80V to bout 7.00V.

The problem is that the potentiometer inputs are not supposed to go above VCC for the chip. They likely have protection diodes, which will limit the ADJ pin to 5.7V or so. If you add 1.25V to that, then you have 6.95V or about 7V.

The problem is that the potentiometer inputs are not supposed to go above VCC for the chip. They likely have protection diodes, which will limit the ADJ pin to 5.7V or so. If you add 1.25V to that, then you have 6.95V or about 7V.

Ok - well, a ULN2803 would be what you would want to use if you didn't have a discreet Darlington or a couple of NPN transistors lying around - like 2N2222's, 2N3904's, etc. and a current limiting resistor. But unless you had use for a few more of the channels, it would be kind of a waste.

A transistor or Darlington will amplify the current that your digital pot can output.
Connect the A divider to your Vdd (that you have marked Vcc), the B divider to ground, and the W(iper) to a resistor of at least 4.4k Ohms. (5v Vdd -0.6v base-emitter drop =4.4v; 4.4v/1mA max pot current = 4.4k Ohms) The other side of the resistor you can use to supply current to the base of an NPN transistor or Darlington. The collector current of the transistor will be the current supplied by the Wiper output times the hFE of the transistor or Darlington. A 2n2222 or 2n3904 might have an hFE of 100 to 300. A Darlington would have upwards of 1,000; as the gains of the two transistors are multiplied.

You connect the collector of the transistor/Darlington to the LM317 ADJ pin and the emitter to ground.

If you're going to use a ULN2803, it already has a 2.7k Ohm limiter in the base circuit. So, instead of a 4.4k resistor, you would need an additional 1.7k Ohm resistor.

It's OK to go up a little in the resistance, for example 4.4k to 4.7k or 1.7k to 1.8k. Don't go down though, unless you like to burn out your components early.

You're currently using a 220 Ohm resistor from the output to the adj terminal. An LM317 requires a minimum of 10mA current to guarantee regulation. This is why they usually show a 120 Ohm resistor used for that purpose. If you will never operate your supply with less than a 4.4mA additional load, you can use the 220 Ohm resistor, otherwise you should use a 120 Ohm resistor.

The circuit does not control the very high voltage gain of the darlington driver.
It is designed to be an on-off switch, not a linear amplifier.
Its conduction changes when the temperature changes and every IC will be different.

Audioguru is right. What you need is a digital pot driving a DC amplifier with a gain of about 4. That way the pot could put out a voltage of 0 - 5V and the DC amplifier will put out a voltage of 0 - 20V. If you use an opamp circuit and limit yourself down to 7V output, you could even use the output of the LM317 to power the opamp.

The opamp would just need to be able to run from 7-20V and preferably be rail to rail output. That should be pretty easy to find.

Audioguru is right. What you need is a digital pot driving a DC amplifier with a gain of about 4. That way the pot could put out a voltage of 0 - 5V and the DC amplifier will put out a voltage of 0 - 20V. If you use an opamp circuit and limit yourself down to 7V output, you could even use the output of the LM317 to power the opamp.

The opamp would just need to be able to run from 7-20V and preferably be rail to rail output. That should be pretty easy to find.

Well then, here's another option; replace the LM317 with an LM375, which is a power op amp by National Semiconductor. You won't get the thermal overload protection the LM317 has, but you'll get more current output than the LM317 could produce. The stability of the output will largely depend upon the stability of your 5v supply. A 10pF cap has been added to the W output of your digital pot, represented by R1. The 10pF should actually be between 100pF and 500pF. There also needs to be a 1 Ohm damping resistor between the output of the opamp and C1 to prevent oscillation.